1. Technical Field
The present invention relates to an electric power steering device having an electric motor for giving to a steering system, an auxiliary steering force based on steering torque acting on the steering system.
2. Related Art
As the electric power steering device of this type, a device has been known that includes a driving circuit having a power source connected between input terminals of a bridge circuit. The bridge circuit is formed by connecting, for instance, four semiconductor elements in an H bridge form. A motor driving circuit having a motor for giving an auxiliary steering force to a steering mechanism is connected between output terminals. A driving control unit drives the semiconductor elements of two pairs of semiconductors, each pair including two semiconductors and the two pairs opposing the two arms of the H bridge circuit. The semiconductor elements of the first arm are driven by a PWM signal of a first duty ratio determined on the basis of an auxiliary steering command value calculated in accordance with a steering torque and a current control value calculated from a detected motor current value. The semiconductor elements of the second arm are driven by a PWM signal of a second duty ratio defined by the function of the first duty ratio (for instance, see Japanese Patent Unexamined Publication JP-A-8-337172 (page 2, FIG. 3 and FIG. 5)).
However, in a usual example described in the JP-A-8-337172, a battery is employed as the power source of the motor driving circuit. Thus when the voltage of the battery falls, an adequate voltage between the terminals of the motor cannot be ensured. Therefore the auxiliary steering force generated in the motor is lowered or the motor is stopped to stop the auxiliary steering force. Thus, a steering operation of a steering wheel is heavy. Especially, in the electric power steering device of a high output, when a wheel turning operation is carried out under a stopping state, an unsolved problem arises that the steering wheel is hardly driven.
Further, when a steering operation is carried out by using a battery whose deterioration is advanced, the internal resistance of the battery increases. As a motor current is increased, the voltage of the battery is markedly lowered. As a result, an unsolved problem arises that voltage between the terminals of the motor cannot be sufficiently ensured and the auxiliary steering force is lowered or a steering assistance is stopped, so that the steering operation of the steering wheel is heavy.